The present invention is directed to improved methods of plating metal on an organic polymer surface, and to improvements in the quality of metal plated organic polymer substrates.
Ever since the structural plastics have replaced metal in the enclosures used in electrical appliances, microwave ovens, business machines, and other electrical/electronic products, manufacturers have had to overcome problems caused by electromagnetic interference in general (EMI) and radio frequency interference (RFI) in particular. The Federal Communications Commission (FCC), since 1983, requires that the electrical products not exceed certain specified EMI/RFI levels. The FCC requirements have been codified in the FCC regulation CRF 47, Part 15, Subsection J. The FCC requirements are met by reducing the EMI/RFI emission from the electrical/electronic products by providing a shielding. With the increased sensitivity of newer, higher-speed, and higher-frequency circuits plus a continued proliferation of electronic devices worldwide, EMI shielding problems are becoming more demanding. This has placed greater emphasis on high signal attenuation by the shielding medium.
The EMI shielded enclosures are also used to protect delicate electronic/electrical circuitry and components enclosed within the enclosure from damage by external sources such as static electricity or man-made high intensity EMI emission produced by an atmospheric nuclear explosion.
Electromagnetic and radio-frequency interference are often referred to as electronic noise. Electronic noise may occur naturally from sources such as lightening or static electricity or from man-made sources such as radio signal, radio games, computers, calculators, cash registers, electrical motors, automobile ignition systems, and all kinds of appliances, especially those that incorporate electronic components. A well-shielded enclosure enclosing the electrical components is often the quickest and most cost-effective way to suppress man-made electromagnetic noise.
Enclosures for electrical components having metal cases, metal foil claddings, wire mesh screens, applied coatings, magnetic materials, and a variety of alternative approaches have been tried. However, because of their cost advantages and ease of processing, plastic enclosures having metallized coatings have emerged as the dominant choice.
Printed circuit boards have become the dominant vehicle for mounting and interconnecting electronic components in order to manufacture a desired electronic circuit. The printed circuit board usually consists of a sheet of a dielectric substrate constructed from various filled or unfilled synthetic materials. The substrate is provided with a pattern of thin metal foil which functions as a conductive path on one or both sides. The paths or "traces" are usually formed of a conductive material such as copper, palladium, nickel or gold. The traces collectively define all of the electrical connections between components on the board, and are routed between the appropriate locations on the board.
Thermoplastics materials such as polycarbonates are particularly suitable for printed circuit board substrates because of their strength, heat resistance, dimensional stability, and ease of moldability. However, polycarbonate substrates are not easily provided with a strongly adherent metal trace. The printed circuit, i.e., the plated metal conductive path, can be damaged or separated from the substrate during the subsequent manufacturing steps or during use of the circuit board. substrates can be then easily electroplated to a desired thickness, either with the same metal or with a different metal or alloy. Thermoplastic materials such as polycarbonates are particularly suitable for making EMI shielded enclosures or housings due to their high impact strength, heat resistance, dimensional stability, and ease of moldability. However, polycarbonate substrates can not be easily electrolessly plated or coated with a strongly adherent conductive metal layer.
Several attempts have been made to increase the adhesion of a conductive metal layer to polycarbonate substrates. Adhesion is generally measured as a "peel strength", i.e. a force required to peel an adherent metal layer from an underlying substrate under controlled conditions. The controlled conditions are specified in The American National Standard Test titled ANSI/ASTM D 3359-76, Measuring Adhesion by Tape Test.
One of the prior art methods for improving adhesion involves grit blasting the surface to provide a roughened profile on which the subsequently-applied metals can be anchored. Other methods call for the use of chemical swelling agents or penetrants to swell the surface prior to the application of a metal layer.
While such methods do increase adhesion, they are often not entirely satisfactory for several reasons. Such techniques result in structural degradation of the polymer molecules disposed along the surface that results in decreasing the tensile as well the impact strength of the underlying polymer substrate. The aforementioned structural degradation results from the swelling and cracking steps to which the entire substrate material is exposed, especially in those instances in which the polymer contains fillers. Additionally, such surface preparations cause crack formation and propagation at highly stressed areas such as at sharp corners or edges of the enclosure being shielded. The presence of such cracks invariably results in a poorly shielded enclosure. As a result, at present, no viable and cost effective process of preparing the surfaces of an EMI/RFI shielded plastic enclosures for an electroless metal plating exists.
Currently, a number of approaches have been tried to solve the problem of applying metallized coatings onto the plastic enclosures or substrates. One involves adding an electrically conductive material to a polymer composition from which a shielded enclosure is molded or formed. However, this method is limited by the type and the amount of conductive additives that can be incorporated into the polymer composition before the physical properties of the composition begin to deteriorate to unacceptable levels. Some work has been done in producing parts made from inherently conductive polymers, such as polyarylenes; however, these materials are intrinsically unstable.
A second approach involves the use of metal-loaded paints. Silver was popular before its rise in price. The paint industry has investigated all types of substitutes, including carbon black, copper and nickel. The most promising paints appear to be those loaded with nickel or copper. However, painted EMI shields are fraught with a number of problems such as chemical attack by the paint solvent on the underlying substrate, difficulty in environmentally safe disposal of the paint waste, flaking, scratching or scuffing of the painted surface, difficulty in controlling the thickness of the painted surfaces, and the high cost of a painting operation.
Several attempts have been made to produce effective EMI shields at a reasonable cost. One of the most effective and promising methods involves electroless plating of the surfaces of the EMI shielded enclosures. Electroless or autocatalytic plating is defined as a deposition of a metal or alloy coating on a suitable substrate by a controlled chemical reduction that is catalyzed by the metal or alloy being deposited. Thin Film Processes (1978) Vossen, J. and Kern, K., Page 213, lines 5-7. A great advantage of the electroless plating solutions is their ability to deposit conductive metal films or layers on the properly prepared non-conductors and their ability to uniformly coat any plateable objects. Electroless plating permits the deposition of pure-metal films onto a prepared molded part surface. Electrolessly metal plated surfaces of the
Therefore the primary object of this invention is to provide a method of applying highly adherent metal layers on a surface of an aromatic polymer substrate by chemically modifying the surface.
Another object of this invention is to provide a metal plated aromatic polymer substrate in the form of an EMI shielded enclosure wherein the noise generated by the enclosed electrical/electronics components is reduced to a desired level by the shielded enclosure.
Yet another object of the present invention is to provide a polycarbonate substrate having nitrophenol polymer chain ends on the polymer chains exposed along the surface of the substrate whereby the surface acquires a hydrophilic characteristic necessary for an electroless metal deposition.
Still another object of the present invention is to provide a printed circuit board comprising an electrically conductive electrolessly plated metal trace pattern disposed on a chemically modified surface of an aromatic polymer substrate, the pattern having an improved adhesion to the surface of the substrate.